1. Field of the Invention
This invention relates to a method of smoothing the surfaces of a disk-like recording medium. This invention particularly relates to a method of uniformly smoothing both surfaces of a double-faced flexible recording medium.
2. Description of the Prior Art
As techniques for smoothing the surface of a magnetic recording medium, the following methods have heretofore been known widely:
(1) A method wherein the dispersant and the dispersing method used at the step of preparing a magnetic coating solution are improved to form a magnetic layer having a relatively smooth surface at the step immediately after the coating.
(2) A method wherein magnetic layers of recording media after coating and drying are contacted with each other and moved at high speeds with respect to each other to rub, grind and smooth the surfaces of the magnetic layers.
(3) A method wherein the magnetic layer surface is rubbed and ground with fur of animals, plastics, metals, ceramics, or the like.
(4) A method wherein the magnetic layer is smoothed by use of press rolls which are called the supercalender rolls.
However, the conventional methods described above present the problems as described below. Namely, in the method (1), the electromagnetic transducing characteristics, particularly the sensitivity and the signal-to-noise ratio obtained are not satisfactory. In the method (2), the drop out due to chipping of the magnetic layers by the grinding is so increased as to make it impossible to put the method into practice. In the method (3), it is impossible to conduct the surface smoothing required for a high-density recording medium. In the method (4), surface smoothing is conducted by passing a recording medium several times between a metal roll and a plastic roll. In this method, since the nip pressure of the supercalender rolls is high, large loads are exerted on the rolls and roll noise occurs when the rolls cannot withstand high pressures. Further, the middle portions of the rolls are thermally expanded due to heat generated by the rolls when they are rotated in the pressed condition and/or due to heat for raising the roll temperature to a value within the range of 40.degree. to 80.degree. C. for the purpose of improving the smoothing effect. In this case, the pressing force of the rolls becomes uneven and, therefore, the thickness and/or smoothness of the magnetic recording medium obtained becomes uneven. In the case of a resilient roll, the hardness of the roll becomes uneven or the roll is cracked when the condition as described above continues for long periods.
In the cases of tapes wherein only one tape side is used for recording, for example, video tapes, audio tapes, and computer tapes, the purpose of smoothing the magnetic layer surface can be accomplished by conducting calendering with the magnetic layer surface contacting the surface of the metal roll of the aforesaid supercalender rolls. Therefore, the supercalender, including various improved types, is widely used.
However, in the case of disk-like recording mediums such as floppy disks, the double-faced type is mainly used to satisfy the need for high density recording. When smoothing of the surfaces of the double-faced floppy disk is conducted by using the conventional supercalender, one surface of the double-faced floppy disk comes into contact with a resilient plastic roll. Therefore, the magnetic layer surface on the side contacting the resilient roll is not so smoothed as the magnetic layer surface on the side contacting a metal roll due to slight deflection and the surface condition of the resilient roll. As a result, many defective products exhibiting drop-out are produced.
In order to eliminate the aforesaid drawback of the supercalendering method, it has been proposed to abrade and smooth the surfaces of punched-out disks one by one by use of an abrasive. However, abrasion cannot fill in or eliminate fine gaps in the magnetic layer which are generated at the coating step and which cause drop-out or deteriorate the electromagnetic transducing characteristics. On the other hand, by the supercalendering, since the magnetic layer is pressed, it is possible to fill in the fine gaps in the magnetic layer.